The generation of superoxide anion by the UVA irradiation of human lens proteins.

In this study we report the generation of superoxide anion and hydrogen peroxide by a water-insoluble protein fraction from aged human lenses in response to UVA light. Irradiation with 1.5 kJ cm-2 of UVA light ( > 338 nm) over a 1 hr period caused the formation of 20 +/- 0.1 microM superoxide radical and 37 +/- 0.5 microM hydrogen peroxide. A linear photolysis of SH-groups (21 nmol ml-1, 26%). His (117 nmol ml-1, 26%) and Trp (72 nmol ml-1, 27%) residues was seen following 60 min of irradiation. The addition of SOD, however, had no effect on the photolytic destruction of any of these amino acid residues. Incubation of the human WISS proteins and bovine alpha-crystallin in the presence of 43-49 microM of O2- generated in a xanthine oxidase/hypoxanthine system over a 1 hr period, caused no loss of histidine, little or no loss of tryptophan and loss of 7-9 nmol ml-1 of sulfhydryl groups with both proteins. This argues that O2- can only account for the destruction of at most 4-8 nmol SH-groups in human water-insoluble proteins following 1 hr of UVA irradiation.